In the art of fluid transfer control, particularly as it applies to the petroleum industry, one of the more common control devices is an overfill sensor for determining when the fluid being transferred into a container, such as a petroleum tanker truck compartment, has exceeded a predetermined level. An output signal from such a probe indicates an overfill risk, and may be used by a fluid transfer controller to discontinue fluid flow into the compartment. In this way, overfilling of the compartment, which is particularly hazardous when dealing with flammable liquids such as gasoline, can be avoided. Such a probe 100 is shown schematically in FIG. 1, which shows a partial cross-section diagram of a tanker truck compartment 102, which is being filled with a fluid 104. The probe 100 is connected by wires 108 to an overfill prevention circuit, which is not shown in FIG. 1. Typically, a well 106 is formed around the top of probe 100 in order to contain any fluid 104 that might leak out around the probe 100.
One type of overfill probe that is known in the petrochemical industry makes use of an optical signal generated by a light source, such as a light emitting diode, which signal is coupled into a medium having a relatively high index of refraction, such as a glass or translucent plastic. This medium is specially shaped and commonly referred to as a “prism.” The shape of the prism provides multiple surfaces at the interface between the prism material and an external environment, and these surfaces are aligned so as to cause an internal reflection of the optical signal coupled into the prism when the prism is surrounded by air. This internal reflection directs the optical signal toward a photodetector that generates an output signal which indicates that the optical signal is being detected.
A schematic illustration of this prior art probe design 200 is shown in FIG. 2. In the plane of the optical signal path 202, the prism 204 has a triangular cross section. The optical signal is generated by light source 206. When the prism 204 is surrounded by air, the optical signal is reflected at two interfaces between the prism material and the surrounding air, and redirected toward photodetector 208 following the path 202. The photodetector 208 generates an electrical output signal that indicates that the optical signal is being detected. This optical signal is directed to components on a printed circuit board that is located in a probe housing 210 and is surrounded by a potting material 212.
The prism 204 of FIG. 2 is mounted in a prism holder 216 that has properly positioned holes for receiving each of the light source 206 and the photodetector 208, and a partial cutaway region for receiving the prism 204. The prism holder 216 may comprise an elastomer seal and may have a potting compound 218 adjacent to it to help seal the internal components from the external environment. The prism holder 216 helps to maintain the prism, light source and photodetector in an appropriate relative alignment.
When the fluid 104 in the compartment 102 rises high enough to contact a prism surface at a location where the optical signal is incident, the prism/air interface becomes a prism/fluid interface, and the fluid has an index of refraction much closer to the prism material than does air. According to Snell's law of refraction, (well-known in the art of optical design) the angle of incidence of the optical signal at the prism/fluid interface now results in the transmission of the optical signal through the interface due to the similarity of the relative indices of refraction. As a result, the signal is no longer detected by photodetector 208, and the corresponding change in the photodetector output signal is detected by conventional signal processing electronics (not shown in FIG. 2) and used as an overfill warning indicating that loading of the compartment 102 should be discontinued.
Overfill probes of this type may be subjected to a particularly harsh environment. If the compartment contains gasoline or other fuels or harsh chemicals, the probe may be subjected to corrosive vapors. In addition, operating conditions for the compartments often include a wide range of temperature changes. Such changes can put a variety of stresses on the probe that could ultimately lead to its failure. A failure of the probe can cause a false overfill signal to be generated, which prevents fluid from being loaded into the compartment, despite the fact that the compartment may be empty. If this happens, it may be necessary to clean or replace the probe in the field resulting in significant downtime.